The present invention relates to a battery power source device comprising the function of protecting a rechargeable battery from excessive discharge current or a state of excessive discharge, and to an electromotive device using such a battery power source device.
When a rechargeable battery is allowed to enter a state of excessive discharge, or is used at an excessive discharge current, battery characteristics are degraded and battery lifetime is markedly reduced. Therefore, an excessive discharge prevention circuit and/or excessive current prevention circuit are provided, these circuits being integrated with the rechargeable battery to constitute a battery power source device, such as a battery pack, or the like. The aforementioned excessive discharge prevention circuit provides control whereby discharge is halted when a voltage equal to or lower than a discharge halt voltage at which discharging must be stopped is detected. The aforementioned excessive current prevention circuit detects an excessive current flowing through the discharge circuit based on voltage, and it performs control whereby discharge is halted if a voltage corresponding to an excessive current is detected.
However, in cases where an inrush current arises when the power is switched on in a device connected to the battery power source device, the voltage of the rechargeable battery is reduced as a result of this inrush current, and if the voltage falls to a value equal to or lower than the aforementioned discharge halt voltage, then the excessive discharge prevention circuit will shut off the discharge circuit, and the device will enter a non-operational state. This situation also applies similarly to an excessive current prevention circuit in that discharge will be halted if the inrush current is detected as an excessive current.
Japanese Patent Laid-open No. (Hei)9-266014 discloses the construction of a battery pack which prevents malfunction of an excessive discharge prevention circuit due to the occurrence of inrush current. In this construction, a time-constant circuit is provided on the input side of a battery voltage detection circuit, in such a manner that voltage reductions of short time period caused by inrush current are not detected. Moreover, it is also disclosed that by providing a timer function at the output side of the aforementioned detection circuit, the detection output is halted during voltage reductions of short time period.
Furthermore, Japanese Patent Laid-open No. (Hei)6-303728 discloses a construction for preventing malfunction of an excessive current preventing circuit due to inrush current. In this construction, a capacitor is provided on the input side of a control circuit which halts the discharge circuit when excessive current is detected, and therefore, since the voltage for detecting excessive current is not generated in the control circuit until the capacitor has become charged, any inrush current of short time duration will not be detected.
If a battery power source device is used in a device generating significant load fluctuations, such as an electric power tool, or the like, then the rechargeable battery will be exposed to harsh operating conditions.
Inrush current is generated repeatedly due to frequent ON/OFF switching operations, and the rechargeable battery must also be protected from excess load conditions, or high-temperature conditions, due to motor locking operations, or the like. Such situations cannot be managed satisfactorily simply by means of a control circuit for preventing malfunction due to inrush current according to the prior art constructions described above. In the case where an alkaline rechargeable battery such as a nickel metal hydride battery is used, because of the low output voltage (nominal voltage : 1.2V) of the battery, a battery power source is constituted by connecting a plurality of cells in series. However, if there is variation in battery capacity between the plurality of cells, then when the cell having the smallest capacity has completely discharged, it will start to be charged inversely by the other cells, thereby provoking breakdown of the battery and degradation of battery characteristics.
It is an object of the present invention to provide a battery power source device whereby shortening of battery lifetime is prevented by means of a control structure which prevents a battery from being used in predetermined unfavorable conditions.
In order to achieve the aforementioned object, the present invention according to a first aspect thereof provides a battery power source device comprising:
a rechargeable battery, wherein a discharge circuit is formed; and
discharge control means for controlling discharge of said rechargeable battery, wherein
said discharge control means measures the voltage and discharge current of said rechargeable battery a prescribed number of times at prescribed time intervals,
detects a battery voltage value and a discharge current value for each unit consisting of said prescribed number of measurement operations from running averages for said unit of measurement operations,
detects the battery temperature of said rechargeable battery, and
shuts off the discharge circuit of said rechargeable battery, if said battery voltage value, discharge current value and battery temperature assume any one of the states (1), (2), (3) described below:
(1) if a state where said battery voltage value is less than or equal to a predetermined undercut voltage value is detected consecutively for a prescribed number of measurement operations or more;
(2) if a state where said discharge current value is equal to or greater than a predetermined excessive current value is detected consecutively for a prescribed number of measurement operations or more;
(3) if said battery temperature is equal to or greater than a predetermined temperature.
According to the above described first aspect of the present invention, since the battery voltage and discharge current are measured at prescribed time intervals and are detected by means of the running average for each prescribed number of measurement operations, it is possible to detect the voltage and current in a suitable manner, even when the rechargeable battery is being used in a state where there are severe fluctuations in load and the voltage and current vary significantly, as in an electric power tool. Since an excessive discharge state of the rechargeable battery is identified if a state where the detected battery voltage value is equal to or lower than a predetermined undercut voltage value is detected continuously for a prescribed number of measurement operations or more, it is possible to shut off the discharge circuit, thereby halting battery discharge, in such circumstances. Consequently, the rechargeable battery can be protected from degradation due to excessive discharge. Moreover, if a state where the detected discharge current value is equal to or greater than a predetermined excessive current value is detected continuously for a prescribed number of measurement operations or more, then it is judged that the discharge current is exceeding the rated current of the rechargeable battery, thereby causing degradation of the rechargeable battery, and hence discharge can be halted in such circumstances by shutting off the discharge circuit. Moreover, if the battery temperature is equal to or greater than a predetermined temperature value, then this may cause deterioration of battery characteristics and lead to degradation or damaging of the battery, and hence discharge can be halted in such circumstances by shutting off the discharge circuit.
The rechargeable battery in the battery power source device of the present invention is constituted by a plurality of cells connected in series. According to a second aspect of the present invention, the discharge control means measures the respective unit voltages of each cell or each block of a plurality of cells, in addition to the voltage and discharge current of the rechargeable battery, and, respectively detects the unit voltage values, the battery voltage value and discharge current value for each unit consisting of a prescribed number of measurement operations from the running averages for the unit of measurement operations. The discharge circuit of the rechargeable battery is shut off, if the battery voltage value, discharge current value and battery temperature assume any one of the above described states (1), (2), (3), or if a state where the difference between the smallest value and the next to smallest value of the respective unit voltage values is equal to or greater than a predetermined allowable value is detected consecutively for a prescribed number of measurement operations or more.
According to this second aspect of the present invention, if a state where the difference between the smallest value and the next to smallest value of the respective unit voltage values is equal to or greater than a predetermined allowable value is detected for a prescribed number of measurement operations or more, then this indicates that the capacity of one of the cells has declined due to discharging of the rechargeable battery, and hence discharge is halted by shutting off the discharge circuit.
The above described battery power source of the present invention can advantageously used in an electromotive device using the battery power source device as a driving power source to drive a motor in rotation. The discharge control means starts measuring the voltage and discharge current of the rechargeable battery from the time that the electromotive device is switched on, and shuts off the discharge circuit if any of the states (1) to (3) described above is detected.
Alternatively, according to a fourth aspect of the present invention, since the rechargeable battery in the battery power source device is constituted by a plurality of cells connected in series, the discharge control means can measure the respective unit voltages of each cell or each block of a plurality of cells, in addition to the voltage and discharge current of the rechargeable battery, and, respectively detect the unit voltage values, the battery voltage value and discharge current value for each unit consisting of a prescribed number of measurement operations from the running averages for the unit of measurement operations. And the discharge circuit of the rechargeable battery is shut off, if the battery voltage value, discharge current value and battery temperature assume any one of the above described states (1), (2), (3), or if a state where the difference between the smallest value and the next to smallest value of the respective unit voltage values is equal to or greater than a predetermined allowable value is detected consecutively for a prescribed number of measurement operations or more.
In any of the first to fourth aspects of the present invention described above, the undercut voltage value can be derived as a voltage value which is specified according to the discharge current value and battery temperature.
Therefore, an undercut voltage value can be determined from the corresponding voltage value by detecting the discharge current value and battery temperature.
Specifically, the undercut voltage value can be set by multiplying a voltage value specified for each cell according to the discharge current value, by the number of serially connected cells. Since a voltage value corresponding to the cell discharge current value is previously obtained, the undercut voltage for the rechargeable battery or for a block of cells can be determined by multiplying this voltage value by the appropriate number of serially connected cells.
Preferably, the battery power source device should adopt a construction wherein, the shut-off state of the discharge circuit is locked and a recharge advice display recommending recharging is implemented, when the battery voltage value or the unit voltage value falls below the undercut voltage value. In a state of excessive discharge, further discharge is not possible and recharging is necessary, and consequently, discharge is halted and recharging is recommended by means of a display.
The battery power source device may further comprise control conditions setting means for changing the setting values for the control operation of shutting off the discharge circuit, as desired. Since the setting values for the control operation can be changed as desired in accordance with the type of equipment and the type of rechargeable battery being used, it is possible to increase the general compatibility of the battery power source device.
The electromotive device may further comprise reset means so that the shut-off state of the discharge circuit can be cancelled by an OFF operation of the electric power tool or a reset operation of the resetting means. A shut-off of the discharge circuit due to a temporary irregular state can be cancelled by such an OFF operation or reset operation, but if the discharge circuit has been shut off due to excessive discharge, or the like, then after cancelling, the discharge circuit will be shut off again.
Preferably, the rechargeable battery should comprise one of a nickel metal hydride battery and a nickel cadmium battery. These types of battery are rechargeable batteries having high energy density suitable for use as driving sources for electric power tools, or the like, and by implementing discharge control, it is possible to prevent degradation thereof and achieve a long operational lifetime thereof.